1. Field of the Invention
The present invention relates to a toner used for developing an electrostatic latent image formed by an electrophotographic method, an electrostatic recording method or the like, and more particularly concerns a toner that requires no oil in a fixing device. The present invention also relates to a pulverized-type toner composition made from at least a binder resin in which wax is uniformly dispersed with particles having a comparatively large size, the wax and a wax dispersant.
2. Description of the Related Art
With respect to the conventional toner fixing system, a heat roll system has been widely used. In the heat roll system, a toner image is made in press-contact with the surface of a heat roll in a melt-heated state. Consequently, one portion of the toner image adheres to the heated roll surface, and is transferred thereon, resulting in a problem of stains on the next sheet to be fixed, that is, a so-called offset phenomenon; and, this offset phenomenon needs to be prevented. In this heat roll system, a separation mechanism such as a separation claw or the like is formed in the roll portion so as to prevent a defective separation in which the sheet for toner-fixing such as paper is wound around the roll after passing through the roll portion. However, in the case of an increased stress due to a high printing speed of a copying machine/printer, or in the case of adhesion of toner clumps onto the leading end of the sheet for toner to be fixed, problems of a defective separation and separation claw scratches are raised.
For this reason, a method which improves the toner mold-releasing property in the heated and fused state by mixing wax such as polypropylene and polyethylene in the toner and a method in which the surface of the heat roll is coated with a resin having a superior separation property such as fluorine-based resin have been proposed. However, waxes such as propylene and polyethylene have poor compatibility to polyester resins having a comparatively strong polarity, which are used in toner manufacturing processes, and the resulting problem is that it is difficult to disperse the wax uniformly. In order to solve such a problem, a technique has been proposed in which a modified wax such as an oxidized-type polyolefin wax having a polar group in the molecular terminal is used to improve the compatibility of the wax to a polyester resin and consequently to improve the dispersibility of the wax. However, although such a technique improves the dispersibility of the wax, the mold-releasing property, which is an inherent function of the wax, is lowered since the modified wax has a higher melt viscosity in comparison with an unmodified matter, with the result that the off-set phenomenon again tends to occur.
The problems with the anti-offset property and separation property are in particular conspicuous in full-color toner. In other words, in comparison with black toner, the full-color toner needs to have a higher heat-melting property upon application of heat for fixing with a lower viscosity, as well as improved gloss, transparency and color reproducibility. However, since a full-color toner using a resin that achieves these required properties tends to have low cohesion between molecules in the heat-melt state, the toner adhesion to the heating roller tends to occur upon passing through the fixing roller, resulting in insufficient separation and high-temperature offset. In order to prevent these insufficient separation and high-temperature offset, an attempt is made to reduce the toner adhesion to the fixing roller by coating the fixing roller with oil. However, the installation of the oil-coating mechanism causes problems of increase costs and a bulky device.
Therefore, an attempt has been made to disperse a wax in a toner by using an additive. For example, the following toners have been proposed: a toner containing a wax dispersion assistant having polyolefin (for example, Japanese Patent Application Laid-Open No. 2001-255690), a toner containing a wax dispersion assistant having a copolymer synthesized by using a styrene-based monomer and polyolefin (for example, Japanese Patent Application Laid-Open No. 2001-249485), a toner using alkylene glycidyl methacrylate as a wax dispersant (for example, U.S. Pat. No. 5,368,970), and toners containing as a wax dispersant a block polymer of ethylene and ethylene oxide (for example, Unisocks 550 (made by Toyo-Petrolite Co., Ltd.)) and/or a block polymer between polyethylene or polypropylene and propylene oxide, acrylic acid, caprolactone, caprolactam, alkyl oxazoline or vinyl chloride; polyethylene block polyester; polyethylene block polyvinylchloride; or polyethylene block polyvinylidene fluoride (for example, U.S. Pat. No. 5,344,737).
However, the total amount of addition of the wax in each of the above-mentioned toners is approximately 4%, and the particle size of the wax dispersant is relatively small; therefore, at the instant of fixing, the wax eluting speed is slow, failing to sufficiently satisfy the separating property from paper in an oilless fixing system in which no oil is applied to the fixing rollers. Since only the slight increase in the amount of wax makes the wax isolate comparatively easily, the cleaning property is lowered with the result that filming is generated on a photosensitive member, and lines, irregularities and black spots (BS) consequently generate on an image, causing problems with image quality. The heat-resistant storing property is lowered to easily cause aggregation in the toner. The degree of gloss in the resulting image is lowered.